1. Field of the Invention
The present invention relates to an output device, a differential output device, a semiconductor laser modulation drive device, an image forming apparatus, and an electronic device that are applicable to high-speed electrical signal transmission; and particularly relates to an output device and a differential output device that are applicable to high-speed electrical signal transmission and are provided with an impedance matching section for preventing impedance mismatch, which causes signal waveform distortion, between a transmission line and a transmitter in electrical signal transmission between integrated circuits (ICs) or between printed circuit boards (PCBs); and a semiconductor laser modulation drive device, an image forming apparatus, and an electronic device that use these output devices.
The present invention further relates to an impedance matching device for preventing impedance mismatch, which causes signal waveform distortion, between a transmission line and a transmitter or a receiver in high-speed electrical signal transmission between integrated circuits; and a semiconductor laser modulation drive device and an image forming apparatus that use the impedance matching device.
2. Description of the Related Art
In order to prevent transmission signals output from an electronic circuit of, for example, an output driver of an integrated circuit, from having waveform distortion due to reflection while being transmitted through a transmission line, impedance matching between an output impedance of the circuit and a characteristic impedance of the transmission line is implemented. Conventionally, termination resistors have been used for such impedance matching.
Simple termination resistors, however, cannot accurately match the impedances due to a process variation. As the transmission speed has increased, problems such as waveform distortion and signal attenuation have arisen due to mismatched impedances. Patent Document 1 discloses a method for accurately matching impedances, wherein the voltage of a transmission line is monitored so as to compare a transition time from an L level to an H level to a reference time, and a comparison result is fed back to a variable resistor used as a termination resistor. Patent Document 2 discloses an impedance matching method for preventing waveform distortion in electrical signal transmission between a semiconductor laser drive section and a semiconductor laser control section.
[Patent Document 1] Japanese Patent Laid-Open Publication No. 2003-8421
[Patent Document 2] Japanese Patent Laid-Open Publication No. 2002-324937
However, with further improvement in transmission speed, if a comparator is connected to an output terminal for monitoring transmission signals, the output waveform is distorted because of the capacitance. This may prevent increasing the transmission speed. Also, time measurement and voltage comparison may result in increasing the size and consumption current of an impedance matching circuit. If a simple switching circuit is used as an output device, the switching speed is determined by a product CR time constant of an output impedance R and a load capacity C and, therefore, a further speed improvement cannot be achieved. Another issue involved in increasing the speed is that because inductance components of an output terminal attenuate signals, it is impossible to transmit a desired output swing.